1. Field of the Invention
The invention relates to an arrangement with a superconductive electrical direct current cable system which includes at least one direct current transmission element composed of two phase conductors which are insulated relative to each other, and a cryostat suitable for conducting a cooling agent in which the direct current cable system is arranged, and which is composed of at least one metal pipe which is completely circumferentially surrounded by a closed layer with thermally insulating properties (WO 2008/148390 A1).
2. Description of the Related Art
In today's technology, superconductive cables include electrical conductors of a composite material which includes ceramic material which, at sufficiently low temperatures, changes into the superconductive state. The electrical direct current resistance of an appropriately constructed conductor is zero with sufficient cooling, as long as a certain voltage is not exceeded. Suitable ceramic materials are, for example, BSCCO (bismuth-strontium-calcium-copper-oxide) as materials of the first generation, or REBCO (Rare-earth-barium-copper-oxide), particularly YBCO (Yttrium-barium-copper-oxide) as materials of the second generation. Sufficiently low temperatures for placing such material into the superconductive state are, for example, between 67K and 90K. Suitable cooling agents are, for example, nitrogen, helium, neon and hydrogen, or mixtures of these materials.
A superconductive direct current cable can be used, for example, as current supply cable on ships, or as connecting cable between points of intersection within a transmission network, or also as connecting cable between different transmission networks. This may be, for example, an arrangement for a high voltage direct current transmission (HGÜ) which can be used for bridging long distances, for example, as sea cables. In comparison to superconductive alternating current cables, superconductive direct current cables have the advantage that no electrical alternating current losses occur. In addition, the direct current cables are not subject to charging currents and no undesired inductive voltage drops occur.
The above mentioned WO 2008/148390 A1 discloses a two phase electrical cable for supplying users with electrical current. It is constructed as a superconductive cable with two phase conductors which are coaxially combined into a unit, and are separated from each other by an inner dielectric. The cable is arranged in a cryostat, which is composed of two concentric pipes between which a vacuum insulation is placed. The cooling agent for producing the superconductive state of the phase conductors is conducted through the cryostat. Cooling of such a cable can be problematic because, for cooling of the superconductive phase conductors, the dielectric may constitute a thermal insulation.